1. Field of the Invention
The present invention relates to a dielectric ceramic composition for microwave which has a high specific dielectric constant .di-elect cons..sub.r and a large Qf value and of which the temperature coefficient .tau..sub.f of resonant frequency (f.sub.0) can easily be controlled.
2. Description of the Related Art
With the recent rapid progress of size reduction and digitalization in electronic technology, particularly in the field of communication components using microwaves, such as comunications satellite systems, mobile phones, and portable personal telephones, a small-size, high-performance and high-reliability microwave resonator has been demanded.
Dielectric ceramics used in microwave resonators are required to have such characteristics as a high specific dielectric constant .di-elect cons..sub.r a large Qf value, and of which the temperature coefficient .tau..sub.f of resonant frequency (f.sub.0) can easily be controlled.
Known dielectric ceramics for microwave applications include BaO--TiO.sub.2 --Nd.sub.2 O.sub.3 --Bi.sub.2 O.sub.3 ceramics (see Japanese Patent Laid-Open No. 102003/81) and BaO--TiO.sub.2 --Sm.sub.2 O.sub.3 --Ce.sub.2 O.sub.3 --Bi.sub.2 O.sub.3 ceramics (see Japanese Patent Laid-Open No. 187162/87). The problems of these conventional materials are: that their Qf value is as low as 4000 to 6500 for use as a dielectric in microwave communication and that their frequency-temperature characteristics are not easy to control in conformity with various cavities.
Japanese Patent Laid-Open No. 169326/95 discloses BaO--TiO.sub.2 --Ln.sub.2 O.sub.3 ceramics (wherein Ln.sub.2 O.sub.3 represents Nd.sub.2 O.sub.3 and Pr.sub.6 O.sub.11) containing Cr.sub.2 O.sub.3, which have a specific dielectric constant .di-elect cons..sub.r of 91 to 93. However, their Qf value is low as 5700 to 6000, and the temperature coefficient .tau..sub.f of resonant frequency (f.sub.0) is from 6 to 9 ppm/.degree. C. Therefore, they are inapplicable to filters whose cavities are made of iron or copper and which have a large .tau..sub.f.